The present invention relates to the rolling of assembled metal slabs and, more particularly, to methods for increasing slab rolling yields and rolling mill efficiency by minimizing shearing and crop losses in the rolling of assembled slabs. This favourable increase in material yield and rolling efficiency is achieved by a new slab geometry formed in one or both edges of the slab. The slab geometry is formed by machining or during casting. The invention is most advantageously applied to the manufacture of aluminium rolled products.
A widely used method of manufacturing aluminium plate, sheet and foil products initially involves the vertical semi-continuous casting of slabs which includes a bottommost leading end, referred to in the art as the “butt” of the slab. The butt is formed as the liquid metal solidifies on the movable bottom block or starter block which is in the open bottom of the mould. The bottom block continuously moves downwardly and away from the mould as the solidified metal slab exits at the open end of the mould at the location previously occupied by the bottom block. The sidewalls of the mould and the sidewalls of the solidified slab exiting the mould are sprayed with water to increase the solidification rate. This casting technique is referred to as direct chill or “DC” casting.
The slabs may then be scalped to remove as-cast surface imperfections and further homogenized by heating in a furnace to provide a uniform chemistry across the slab cross-section prior to laminating with a second or further slab. The second slab is commonly of an aluminium alloy of different composition welded to the core, often a braze material or a cladding aimed at improving the corrosion behaviour.
In order to process the thus treated slabs to useful end products, such as sheet, plate, foil or the like, the slabs are preferably welded together at the edges and thereafter heated to a desired rolling temperature and subjected to a plurality of hot rolling passes in a hot rolling mill to laminate the slabs, followed by cold rolling, whereby a clad materials results. The core slab may be assembled to several further slabs, located on either side of the slab core or on the same side, and often made from a softer metal than the core, such as a braze material with a high silicon content. The slabs forming the clad may also be made from aluminium alloys aimed at improving the corrosion resistance of the rolled strip.
The resulting material is useful as a heat exchanger strip or plates, e g as a fin or tube material as well as for evaporator and header plates.
The free surfaces existing on a slab of finite width, thickness and length allow non-uniform rolling deformation to occur in the length and width dimensions during hot rolling. This non-uniform deformation causes an elongation of the slab in the centre region thereof which forms a convex, longitudinally extending “tongue” condition at the edges thereof, particularly in aluminium slabs which are roughed down in reversing mills, usually without the use of side or edge rolls. Formation of a tongue condition is, however, not uncommon in the rolling of aluminium even in mills equipped with edge rolls. When rolling a clad material, such as a slab for heat exchanger strip, the clad is often softer than the core, thus deforming more easily, making the problem of non-uniform deformation more severe. The force acting on the slabs will be higher at the edges running parallel to the rolling direction, due to the elastic deformation of the rolls. This leads to the formation of an uneven clad close to the edges of the slab and to an increased shearing loss.
In JP59027701 the edges of the side faces of a steel slab has continuous corrugations in order to minimize flaws and reduce margins for trimming. This measure has not shown to be efficient for clad slabs.
The aforesaid non-uniform deformation phenomenon is equally severe in the length direction of the slab leading to another condition referred to in the art as “fold over”, “overlap” or “alligatoring”. These objectionable conditions at the ends of the slab grow worse as rolling continues and must eventually be removed by a crop shear to permit further rolling to continue.
It is (also) well-known that the overlap causes an internal lamination crevice in the metal which grows during rolling and will result in unsound plate and sheet products unless it is removed by shearing.
The use of a clad material also makes recycling of the scrap more difficult, due to the mixed composition in the assembled material.
Previous experimental work has been undertaken JP61262456 in an effort to reduce slab rolling shearing and crop losses by tapering the ends of the slab by deforming the edges of a slab by issuing inclined rolls to increase the yield at rolling. The clad is cast onto the deformed core to create a thicker clad at the edges of the slab, thereby assuring a thick enough cladding also at the edges. The problem with unevenness of the cladding will however remain.
WO01/94050 discloses an aluminium slab with a reduced cross-section in the ends of the slab to reduce crop losses. The problem of uneven clad deformation during rolling is not assessed.